Page 38 - Climate Control Magazine September 2018

Chillers
OPTIMISATION OF
CONDENSER WATER
For every 1°C decrease in CW, chiller compres- sors consume 2–3 per cent less energy for fixed- speed compressors and 4–5 per cent less for vari- able speed compressors.
Cooling towers are typically designed to pro- duce CW at temperatures that are 3–4°C higher than the prevailing ambient wet bulb tempera- ture. This temperature difference is called the cooling tower ‘approach’.
CW temperature reset is based on the modu- lation of the cooling tower fan speeds to track the ambient wet bulb temperature, which pro- vides the lowest possible condensing conditions.
Optimisation is essentially a balancing exercise between reduced chiller energy consumption due to the lower condensing temperature) and higher fan energy consumption at the cooling towers.
This optimisation should be assessed on a sys- tem-by-system basis, taking account of the fol- lowing factors:
• Resetting CW temperature should take into ac-
count the minimum required CW temperature for the given type of refrigeration compressor, as nominated by the equipment manufacturer. CW that is too cold can create an excessively low pres- sure differential between the condensing and evaporating sides of a refrigeration cycle, causing difficulties with refrigerant flow through expan- sion devices and oil transfer to the compressor.
• For the majority of cooling towers, any attempt to
get closer than within 3–4°C approach of the pre- vailing ambient wet bulb temperature (for exam- ple, wet bulb temperature plus 2°C) will result in a significant increase in energy consumption of the cooling tower fans, with little or no net benefits.
• For the successful application of the CW tem- perature reset control strategy, it is essential that the minimum approach temperature for a given brand and model of cooling tower is determined through seeking this information from the manufacturer.
Systems with multiple towers and towers with multiple fans should have all towers/fans con- trolled together. Running multiple fans together at part-load is more energy efficient than running individual fans at full speed, or cycling fans on/off.
Tip: The most cost-effective option should be assessed on a case-by-case basis.
ENERGY-SAVING POTENTIAL, COSTS, BENEFITS AND RISKS HHW, CHW, and CW temperature reset is typi- cally a very cost-effective HVAC energy-efficiency improvement, as it requires minimal investment and immediately reduces the energy consump- tion of the HVAC system by reducing its load.
Resetting HHW delivery temperature can save up to 5 per cent of energy consumed by condensing boil- ers, while resetting CHW delivery temperature can save up to 15 per cent of energy consumed by chillers. Resetting CW delivery temperature can save up to 15 per cent of energy consumed by chillers.
APPLICATION NOTES
CHW temperature reset is applicable to both air- cooled and water-cooled chillers. Decent savings are available for most types of compressors and significant savings are possible with modern variable speed compressors.
CW temperature reset is applicable for water- cooled chillers and water-cooled DX equipment. Significant savings are possible with modern
variable speed compressors.
GETTING STARTED
Current temperature set points should be deter- mined along with the operational characteris- tics and limits of the HVAC equipment of the fa- cility. Reset limits and modulation of the actual heating or cooling of the load based on energy metering, ambient conditions and field modulat- ing valves will need to be determined to inform the implementation of the strategy.
RETROFIT OF ELECTRONIC EXPANSION VALVES
This activity improves the energy efficiency of refrigeration systems. It involves the retrofitting of electronic expansion valves (EEVs) to replace thermostatic expansion valves (TXVs) in vapour compression refrigeration systems that operate with direct expansion (DX) type evaporators.
This opportunity is applicable to DX-type chill- ers and large packaged systems that have TXVs.
Retrofitting EEVs will be more cost-effective when carried out for DX systems that have wide- ly varying load conditions (as opposed to con- stant load) or are being upgraded for variable head pressure control.
Many existing AC systems are likely to have TXVs and there is often a significant energy-effi- ciency benefit from retrofitting systems with an EEV (see Figure 11). EEVs provide better control of the refrigerant flow into the evaporator, with less superheat requirements at the entry to the compressor for its safe operation without risk of damage from liquid refrigerant.
This increases the efficiency of the evaporator and reduces the energy consumption of the compressor. Since TXVs are more likely to exist in older
systems that have R22 (HCFC-22) refrigerant, which is currently being phased out in Austral- ia, it is important to assess the expected lifes- pan of the chiller or AC system when consider- ing this optimisation for implementation. It is important to seek the advice of the manufac- turer of the refrigeration system when carrying out this optimisation.
Retrofitting an electronic expansion valve in a refrigerant vapour compression circuit can result in reduced energy consumption of air conditioning compressors by 10–15 per cent and improved control and reliability of operation of refrigeration systems during part-load conditions.
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